With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric cars and hybrid cars have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary batteries used in large size applications which have excellent storage characteristics. Under these circumstances, the high-energy lithium ion secondary batteries having advantages such as a high discharge voltage and a large discharge capacity have been noticed. In particular, in order to apply the lithium ion secondary batteries to electric tools, electric vehicles or the like in which rapid charge/discharge operations are needed, it has been required that the lithium ion secondary batteries exhibit excellent rate characteristics.
Hitherto, as positive electrode active substances useful for lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known Li2MnO4 having a spinel type structure, LiMnO2 having a zigzag layer structure, LiCoO2 and LiNiO2 having a layer rock-salt structure, or the like. Among the secondary batteries using these active substances, lithium ion secondary batteries using LiNiO2 have been noticed because of a large discharge capacity thereof.
However, LiNiO2 tends to exhibit a low discharge voltage and tends to be deteriorated in thermal stability upon charging as well as cycle characteristics and rate characteristics, and, therefore, it has been required to further improve properties thereof. In addition, when subjecting LiNiO2 to high-voltage charging to obtain a high capacity, there tends to arise such a problem that the structure thereof is broken.
Also, LiMnO2 is excellent in rate characteristics and cycle characteristics, but exhibit a low discharge voltage and a small discharge capacity, and therefore tends to hardly provide a high-energy positive electrode active substance.
In recent years, the positive electrode active substances having a high discharge voltage have been noticed. Typical examples of the known positive electrode active substances having a high discharge voltage include LiNi0.5Mn1.5O4, LiCoMnO4, Li1.2Cr0.4Mn0.4O4, Li1.2Cr0.4Ti0.4O4, LiCoPO4, LiFeMnO4 and LiNiVO4.
Among these materials, LiNi0.5Mn1.5O4 has such a high discharge voltage that a discharge plateau region thereof is present in the range of not less than 4.5 V, and is excellent in rate characteristics and cycle characteristics. Therefore, LiNi0.5Mn1.5O4 has been especially noticed as a next generation positive electrode active substance.
There is very longtime a continuous demand for positive electrode active substances which have a high voltage and a higher capacity from the standpoint of an adequate energy density and are also capable of satisfying cycle characteristics.
Conventionally, there have been attempted various improvements of positive electrode active substance particles having a composition of LiNi0.5Mn1.5O4 (Patent Documents 1 to 7 and Non-Patent Documents 1 and 2).